The present invention relates to a field emitter cell and array, in particular, to an integrally gated, self-aligned field emitter cell and array whose cathode is formed of a recently discovered class of materials of nanotubes and nanowires, collectively referred to as nanofilaments.
Field emitters arrays (FEAs) are naturally small structures which provide reasonably high current densities at low voltages. Typically, FEAs are composed of emitter cells in the form of conical, pyramidal, or cusp-shaped point, edge or wedge-shaped vertical structures. These cells are electrically insulated from a positively charged extraction gate and produce an electron beam that travels through an associated opening in the positively charged gate.
The typical field emitter structure includes a sharp point at the tip of the vertical structure (field emitter) and opposite an electrode. In order to generate electrons which are not collected at the extraction electrode, but can be manipulated and collected elsewhere, an aperture is created in the extraction electrode. The aperture is larger (e.g., two orders of magnitude) than the radius of curvature for the field emitter.
Consequently, the extraction electrode is a flat horizontal surface containing an extraction electrode aperture for the field emitter. Such an extraction electrode is referred to as the gate electrode. The field emitter is centered horizontally in the gate aperture and does not touch the gate although the vertical direction of the field emitter is perpendicular to the horizontal plane of the gate. The positive charges on the edge of the extraction electrode aperture surround the field emitter symmetrically so that the electric field produced between the field emitter and the gate causes the electrons to be emitted from the field emitter in a direction such that the electrons are collected on an electrode (anode) that is separate and distinct from the gate. The smaller the aperture, i.e., the closer the gate is to the field emitter, the lower the voltage required to produce field emission of electrons.
The sharp point at the tip of the field emitter provides for reduction in the voltage necessary to produce field emission of electrons. As a result, numerous micro-manufacturing techniques have been developed to produce various sharp tip designs. Current techniques include wet etching, reactive ion etching (RIE), and a variety of field emitter tip deposition techniques.
Effective methods generally require the use of lithography which has a number of inherent disadvantages including a high equipment and manufacturing cost. For example, the high degree of spatial registration requires expensive high resolution lithography.
Additionally, cathode structures include very small localized vacuum electron sources which emit sufficiently high current. However, these vacuum electron sources are difficult to fabricate for practical applications. This is particularly true when the sources are required to operate at reasonably low voltages. Presently available thermionic sources do not emit high current densities, but rather result in small currents being generated from small areas. In addition, thermionic sources must be heated, and thus require special heating circuits and power supplies. Photoemitters have similar problems with regard to low currents and current densities.
Recent advancements in nanotechnology have resulted in the creation of nanofilaments including nanotubes. One such example is carbon nanotubes. These nanotubes behave like metals or semiconductors and can conduct electricity better than copper, transmit heat better than diamond, and are among some of the strongest materials known while being only a few nanometers in diameter. Nanofilaments can have small diameters, ranging down to only a few nanometers. The nanofilaments may be grown to various lengths (e.g., 100-1000 nm) yet their diameter remains uniform. The aspect ratio (length to diameter) is extremely high.
Nanofilaments in the form of nanotubes have a hollow edge which is on the order of a couple of Angstroms thick. The nanotubes may be either single, double, or multiple walled (i.e., one nanotube within a second, third or further nanotube). For a more comprehensive discussion on carbon nanotubes, see xe2x80x9cCarbon Nanotubes Roll On,xe2x80x9d Physics World, June 2000, pages 29-53.
Carbon nanotubes have been proposed as excellent candidates for use as field emitter cathodes due to: (1) the extreme sharpness of their edges and the extremely large aspect ratio, which enable the achievement of low operating voltages; (2) the resistance to tip blunting by residual back ion bombardment due to the uniform wall thickness throughout their height; (3) the relative inertness, high mechanical strength and current carrying capacity; and (4) an inherent current-limiting mechanism in the presence of adsorbed water which retards emitter burn out and destruction by arcing, a problem plaguing the present day FEAs. Nanotubes have been demonstrated in use as a cathode in a cathode lighting element in which the carbon-nanotubes act as the field-emitting cathode.
To be effective emitters, the nanofilaments need to be oriented largely perpendicular to the substrate. Recently, this property has been achieved by growing the nanofilaments on substrates under suitable conditions such as by high temperature chemical vapor deposition (CVD) on catalytic surfaces. For example, CVD has been used to form extremely vertical and uniformly grown carbon nanotubes directly above a metal catalyst substrate of patterned and oxidized iron patches. The resulting nanotubes form an ungated clump electrode which provides a stable field emission over the entire test duration of 20 hours.
On the other hand, high emission current from carbon nanotubes oriented parallel to the substrate has also been observed, which can be attributed to defects on the tube sidewalls. Nanotubes in this orientation can be expected to erode more quickly than those oriented perpendicular to the substrate by residual back ion bombardment.
However, these nanofilament electrodes are not gated and thus, have limited practical use as field emitters. In order to use nanofilaments as a field emitter, one must control the operating characteristics of the nanofilaments, i.e., the turning on and off of small selected groups (i.e. clumps) of nanofilament emitters which comprise an array of emitter cells (e.g. pixels). This control is accomplished by providing a gate electrode, whose applied voltage bias controls the turning on, turning off and the field emission current magnitude. In order to enable low voltage operation, it is necessary to provide a control gate in very close proximity to a group of nanofilament emitters.
One proposed method of forming a gated nanofilament field emitter includes pre-positioning a paste layer of the nanotubes separately on a substrate and assembling a control grid gate assembly to the paste layer of nanofilaments. This and other presently available manufacturing techniques (all non-integral) fail to provide practical (e.g., in terms of functional and economical) gating of nanofilaments, e.g., nanotube, field emitters.
One clear disadvantage of this method is that the resulting gated unit tends to be large when compared to integrally formed conventional field emitter cells, which limits the resolution. As a result of the increased emitter-grid gate separation, these grid-gated emitters require a much higher gate voltage (hundreds of volts as compared to tens of volts for integrally gated emitters) for their operation.
An additional disadvantage with presently available carbon nanotube field emitting cathodes is that the grid-type control gates and nanotube cathodes are not self-aligned with one another because the control grid gate is assembled to the nanotubes after a paste layer of nanotubes has already been formed. As a result, the gate current (e.g.: current intercepted by the gate) tens to be very high which can cause overheating. In addition, this approach generally does not provide precise control and operation of the FEA and in particular, precise control of individual cells forming the emitter array, as compared to integrally formed and self-aligned control gate and cathode design.
In accordance with the present invention, a self-aligned, integrally gated nanofilament field emitter cell and array is provided wherein a nanofilament cathode (in the form of a group or xe2x80x9cclumpxe2x80x9d of nanofilaments) and control gate are formed through the microprocessing techniques of the subject invention, thereby self-aligning the nanofilament cathode with the control gate.
According to one aspect of the invention, a field emitter cell is provided which comprises an electrically conductive substrate layer. An insulator layer is disposed directly upon the substrate layer and an electrically conductive gate layer is disposed directly on the insulator layer. An aperture on the gate layer extends through the insulator layer to the substrate layer. A catalyst layer is applied to a surface conductively associated with the substrate layer. Electrically conductive nanofilaments are grown on the catalyst layer. The group of nanofilaments are electrically isolated from the gate. When the field emitter cell is operational, the group of nanofilaments act as a cathode.
In alternate embodiments, the catalyst layer upon which the nanofilaments are grown is applied to top surfaces of various structures which comprise a post structure, a tip structure, and an obelisk structure extending from the substrate surface.
One advantage of the present invention is that a field emitter cell is provided in which the cathode comprising a group of nanofilament emitters in close proximity to a control gate (electrodes). As a result of this close proximity, in conjunction with the extreme nanofilament tip sharpness, the control gate electrodes use a much lower emitter operating voltage as compared with currently demonstrated nanofilament grid-gate or ungated field emitter designs.
Yet, another advantage of the present invention is the resistance of the nanofilaments to blunting by residual back ion bombardment because the edge will remain at the same sharpness due to the uniform thickness throughout their heights. Yet, another advantage of the present invention is that the carbon nanotube has a relatively clean and inert surface (i.e. no non-volatile oxides), which enhances higher emission stability. Another advantage is that often these nanofilaments either possess or can be tailored to posses significant resistance which, during emission, will lead to an IR (current times resistance, from the equation V=IR where V=voltage, I=current, and R=resistance) drop in the potential between the gate and the emitter, thereby preventing emitter burn-out by limiting the current. Further, carbon nanotubes, in the presence of adsorbed water, provide an inherent current-limiting mechanism which tends to retard emitter burn-out as disclosed in xe2x80x9cCurrent Saturation Mechanisms In Carbon Nanotube Field Emitters,xe2x80x9d Applied Physics Letters, volume 76, no. 3, Kenneth A. Dean and Babu R. Chalamala, Jan. 17, 2000, herein incorporated by reference.
It is an object of the present invention to provide a self-aligned integrally gated nanofilament field emitter cell and array.
It is an another object of the present invention to provide a integrally gated (but not necessarily self-aligned) nanofilament field emitter cell and array.
It is another object of the present invention to provide a field emitter cell and array in which the gate electrode is placed in very close proximity to a group of nanofilament emitters.
It is yet another objective of the present invention to provide a field emitter cell and array in which the cathode is resistant to blunting and surface contamination.
It is yet another object of the present invention to provide a field emitter cell and array with a very low turn-on voltage and that has a stable field emission.
It is yet another object of the present invention to provide a field emitter cell and array that is very economical to manufacture because no precise lithography is required. In fact, when using the method of the present invention, no lithography is required in making the field emitter cell and array if a stamping technology is used to make the masks for the etching of the starting apertures.
Further features and advantages of the present invention are set forth in, or apparent from, the description of preferred embodiments which follows.